Performance evaluation of REOB-modified asphalt binders and mixtures

This study evaluated a limited number, but well-controlled group of asphalt binders of the same performance grade made with a wide range of recycled engine oil bottoms (REOB) contents; 0%, 2.5%, 6% and 15%. The most practical indicator of the possible presence of a considerable quantity of REOB with measureable changes to the rheological and performance characteristics was the difference between the bending beam rheometer (BBR) m-value temperature grade and the BBR S stiffness temperature grade, ΔTcritical. When a binder exhibited a large ΔTcritical, it was associated with larger differences in performance losses depending on the binder tests and to a lesser extent the mixture test. The ΔTcritical performance disruption was made worse by oxidative ageing by means of double pressure ageing vessel conditioning as well as holding the binder at extended low temperatures before testing. The impact of REOB on moisture damage resistance showed higher moisture sensitivity with increasing REOB content, but did not interfere with liquid anti-strip additives. Mixture cracking test results were mixed. Low-temperature relaxation, strength and fracture measured with thermal stress restrained specimen test showed the fracture strength had slight increases or decreases with 2.5% and 6% REOB and could be interpreted as unaffected. Ageing also improved the average strength of these mixes. However, the strength of the highest 15% REOB mix was measurably decreased and made worse by ageing. The impact of REOB on intermediate temperature fatigue cracking performance depended on the ageing condition and whether stress-control or strain-control performance was considered. To minimise risks, a best practice needs to be developed which includes a maximum use level, taking into account both the variability of REOB and the effects on asphalt binders from different sources. Asphalt binder specifications might be refined by placing lower limits to BBR stiffness or a maximum allowable ΔTcritical. More focus should be placed on intermediate REOB levels such as near 10% to the 6% used in this study to better identify performance pitfalls and performance benefits from REOB and the corresponding ΔTcritical.